Heat exchanger with heat transfer augmentation features

ABSTRACT

A heat exchanger includes a plurality of longitudinally-extending first channels and a plurality of second channels fluidly isolated from the plurality of first channels. Each first channels includes a plurality of spiraling internal fins and a plurality of external fins. The internal fins extend from and are integrally formed with the internal walls of the first channel. The external fins connect extend from and are integrally formed with the external walls of the first channels, connecting channels together. The plurality of second channels is defined in part by external walls of the plurality of first channels and the plurality of external fins.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.DE-AR0001342 awarded by United States Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND

Heat exchangers are central to the functionality of numerous systems,including a variety of oil and air-cooling applications, recuperations,and waste heat harvesting for power cycles. These applicationscontinually require increases in heat transfer performance, reductionsin pressure loss, and reductions in size and weight. Current heatexchanger offerings are dominated by plate fin constructions, with tubeshell and plate-type heat exchangers having niche applications. Heattransfer rates decrease as fluid flows down the length of a channel.There are several methods of augmenting heat transfer, one of which isto increase the surface area of a material that a flowing fluidcontacts. Fins are used within channels to increase surface area withoutaltering the external dimensions of the channel itself. However,traditional plate-fin construction imposes design constraints thatinhibit performance, increase size and weight, results in structuralreliability issues, make it unfeasible to meet future high temperatureapplications, and limit system integration opportunities. Other knownenhancement techniques include twisted tapes and wire coils that areinserted into the channel and serve the same function as fins. Simplyincreasing fin size or number of fins to maximize surface area andaugment heat transfer can result in designs that are too heavy andinefficient, and inserts suffer from similar shortcomings as well asincreased wear and tear. There is a need for heat exchanger heattransfer augmentation features that are designed for and able towithstand high pressure and temperature applications usingcharacteristics besides increased fin size, and for designs providingincreased heat transfer performance, reduced pressure losses, andreduced size and weight.

SUMMARY

A heat exchanger includes a plurality of longitudinally-extending firstchannels and a plurality of second channels fluidly isolated from theplurality of first channels. Each first channel includes a plurality ofinternal fins and a plurality of external fins. The internal fins extendfrom and are integrally formed with the internal walls of the firstchannel. The internal fins have a spiraling orientation along theinternal wall. The external fins connect adjacent first channels. Theplurality of second channels is defined at least in part by externalwalls of the plurality of first channels and the plurality of externalfins.

The present summary is provided only by way of example, and notlimitation. Other aspects of the present disclosure will be appreciatedin view of the entirety of the present disclosure, including the entiretext, claims and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of a heat exchanger core.

FIG. 2A is a perspective view of a segment of another embodiment of aheat exchanger core.

FIG. 2B is a perspective view of a fluid flow in the heat exchanger coresegment of FIG. 2A with a channel, internal fins, external fins, and anouter wall removed.

FIG. 3A is a cutaway perspective view of a heat exchanger channel.

FIG. 3B is a perspective view of internal fins of the heat exchangerchannel of FIG. 3A.

FIG. 3C is a cross-sectional view of the heat exchanger channel of FIG.3A taken along the 3C-3C line of FIG. 3A.

DETAILED DESCRIPTION

The present disclosure is directed to an additively manufactured heatexchanger core with channel configurations having various internal andexternal fin arrangements designed to augment heat transfer. Thedisclosed heat exchanger core configurations are applicable tocounterflow heat exchanger designs and are specifically suited forapplication in supercritical CO₂ cycles, which operate at high pressureand depend heavily on heat transfer for cycle efficiency. Channelconfigurations and internal fins in each fluid channel can be additivelymanufactured in orientations, arrangements, and shapes to augment heattransfer. The present application discloses several embodiments ofadditively manufactured channel and internal fin design and arrangementthat utilizes surface area, shape, and orientation to improve rates ofheat transfer.

Additive manufacturing processes can produce highly complex partsquickly and efficiently and can permit modifications to designspecifications of a desired part, for example by modifying CADspecifications, without re-tooling casting or machining equipment usedfor traditional, subtractive manufacturing processes. Additivemanufacturing allows complex design features to be incorporated intoparts where those complex design features had proved infeasible usingprevious manufacturing techniques. While the disclosed heat exchangercores have been developed using direct metal laser sintering, otheradditive manufacturing techniques may be employed, such as, for example,electron beam melting, electron beam powder bed fusion, laser powderdeposition, directed energy deposition, wire arc additive process,electron beam wire, and selective laser sintering, as well as otherpowder bed methods in general. Powder bed methods work well with metalsas well as plastics, polymers, composites and ceramics. Additivemanufacturing allows for the manufacture of channels with complexinternal fin geometries and arrangements that can be integrally formedwith channel walls to provide for uninterrupted heat conduction.

FIG. 1 is a perspective view of a segment of a counter-flow heatexchanger core. FIG. 1 shows heat exchanger core segment 10 with fluidchannels 12 and 14, internal fins 16, external fins 18 and 20, internalwalls 22, external walls 24, and fluids F₁ and F₂. A first fluid circuitis defined by fluid channels 12 and is configured to deliver F₁ along alength of channels 12 as illustrated by the direction of the arrowdepicting a flow of fluid F₁, substantially axially with respect tochannel 12. Internal fins 16 are disposed in a fluid flow path inchannels 12. Adjacent channels 12 are joined by external fins 18 forminga second fluid circuit therebetween. Channels 14 are defined in part byexternal walls 24 of channels 12 and external fins 18. External fins 20extend into channels 14. Fluid F₂ is delivered in an opposite directionfrom fluid F₁ in channels 14 as illustrated by the direction of thearrow depicting a flow of fluid F₂. Internal fins 16 in channels 12 andexternal fins 18 and 20 in channels 14 augment heat transfer betweenfluids F₁ and F₂. Channels 12 and channels 14 are fluidly isolated. Itwill be understood by one of ordinary skill in the art that heatexchanger core segment 10 can be produced in any number of repeatingunits or subunits thereof to produce a heat exchanger core of a desiredshape and size.

Channels 12 extend longitudinally (i.e. axially), as illustrated in FIG.1 . Channels 12 can have a generally circular cross-section. (i.e.,channels 12 can be cylindrical tubes) to accommodate pressurized fluids.In other embodiments, channels 12 can have other shapes to optimizefluid flow dynamics and heat transfer. Channels 12 are configured totransmit a cooling fluid and channels 14 are configured to transmit aheating fluid but in other embodiments the two may be reversed. Inillustrative embodiments, channels 12 can be configured to transmitsupercritical CO₂ and channels 14 can be configured to transmit air.

Internal fins 16 can be disposed in channels 12 to increase convectivesurface area and augment heat transfer between fluids F₁ and F₂.Internal fins 16 can be integrally formed with internal walls 22 ofchannels 12 and can be arranged in a spiraling orientation along theflow length. Internal fins 16 can be connected to internal walls 22 by afillet. The shape and orientation of internal fins is discussed withrespect to FIGS. 3A-3C. The incorporation of internal fins 16 canincrease surface area without altering the external dimensions ofchannels 12 or channels 14. The shape, size, and orientation of internalfins 16 have effects on flow, boundary layer, and heat transfer rate.

Channels 12 are connected to adjacent channels 12 by external fins 18.External fins 18 can extend from and can be integrally formed withexternal walls 24 of channels 12 to provide uninterrupted conductiveheat transfer. External fins 18 are integrally formed with channels 12.External fins 18 can be connected to external walls 24 of channels 12 bya fillet. External fins 18 that connect adjacent channels 12 are longerthan external fins 20 and can extend twice the length of external fins20. In other embodiments, the connecting external fins 18 may be morethan twice the length of external fins 20 or less than twice the lengthto optimize fluid dynamics and heat transfer. As illustrated in FIG. 1 ,four adjacent channels 12 are connected to each other by external fins18 to define each channel 14. In other embodiments, less than four ormore than four adjacent channels 12 can be connected by external fins 18to define channels 14 of different shapes. External fins 20 areintegrally formed with external walls 24 of channels 12 and extend intochannels 14. External fins 20 are disposed between adjacent externalfins 18. As illustrated in FIG. 1 , four external fins 20 extend intoeach channel 14. In other embodiments, more or less external fins 20than those illustrated in FIG. 1 may extend into each channel 14.Channels 12 can have diameters and wall thickness designed forparticular applications. For example, channels 12 can be designed withwall thicknesses and cross-sectional diameters to accommodatepressurized fluids (e.g., supercritical CO₂). In some embodiments, wallsof channels 12 can have a thickness greater than a thickness of externalfins 18 to accommodate pressurized fluids in channels 12.

Channels 14 extend longitudinally and are defined by external walls 22of channels 12 and external fins 18. Each channel 14 is disposed betweenadjacent channels 12 and external fins 18 such that channels 12 andexternal fins 18 surround each channel 14. Heat is transferred betweenfluid F₁ and fluid F₂ by external walls 24 of channels 12 and externalfins 18 and 20. As illustrated in FIG. 1 , external fins 18 are wavy,undulating in the form of a sine wave oscillation along a length ofchannel 12 but disposed perpendicular to an axis of channels 12 suchthat in the cross-sectional view, form a straight line between adjacentchannels 14. External fins 18 together with circular channels 12 form abox-like shape defined by adjacent sides joined by rounded concavecorners. The cross-sectional shape of channel 14 changes along thelength of channel 12 due to the oscillation of the wavy external fins18. For example, the cross-sectional view in FIG. 1 shows external fins18 disposed at a midpoint of channels 12, but the location of theintersection of external fins 18 and external wall 24 changes along thelength of channels 12. In other embodiments, the shape of channels 14may be different, corresponding to alternative shapes and arrangementsof external fins 18 and channels 12.

External fins 20 extend from and are integrally formed with channels 12and protrude into channel 14. External fins 20 can be connected tochannels 12 by a fillet. External fins act as internal fins to channel14. As illustrated in FIG. 1 , external fins 20 can be wavy, undulatingin the form of a sine wave oscillation. The shape of external fins 20can match the shape of external fins 18. External fins 18 and 20 canhave a wavy shape in a form different than that of a sine waveoscillation and in other embodiments. The wavy shape increases thesurface area available for heat transfer without modifying the externaldimensions of channels 14.

The disclosed internal and external fin orientations and configurationscan provide improved heat transfer in counter-flow heat exchangerapplications. The internal fins 16 and external fins 18 and 20 areintegrally formed with channel walls to provide uninterrupted conductiveheat transfer. The spiraling and wavy shapes of the internal fins 16 andexternal fins 18 and 20 augment heat transfer by increasing the heattransfer coefficient. The spiraling and wavy shapes of internal fins 16and external fins 18 and 20 create a more turbulent flow which allowsfor heat transfer through both conduction and convection. The spiralinginternal fins of channel 12 provide advantage for two phase fluid flowby forcing liquid toward the channel wall while vapor collects in thecenter, enhancing heat transfer. The wavy shapes of external fins 18 and20 increase impingement of fluid on fins 18 and 20 to enhance heattransfer. The shape and orientation of the internal fins 16 and externalfins 18 and 20 allows for augmented heat transfer without modifying theexternal dimensions of channels 12 and 14.

FIG. 2A is a perspective view of another heat exchanger core segment.FIG. 2A shows segment 25 of a heat exchanger core, including fluidchannels 26 and 28, internal fins 30, external fins 32, internal wall34, external wall 36, outer wall 38, and fluids F₁ and F₂. FIG. 2B is aperspective view of a fluid flow through channels 26 and 28 with a wallof channel 26, internal fins 30, external fins 32, and outer wall 38removed. FIGS. 2A and 2B are discussed together. It will be understoodby one of ordinary skill in the art that heat exchanger core segment 25can be produced in any number of repeating units or subunits thereof toproduce a heat exchanger core of a desired shape and size.

A first fluid circuit is defined by fluid channel 26 and configured todeliver fluid F₁ along a length of channels 26, as illustrated by thedirection of the arrow depicting a flow of fluid F₁ in FIG. 2 ,substantially axially with respect to channel 26. Internal fins 30 aredisposed in a fluid flow path in channel 26. External fins 32 extendfrom external wall 36 of channel 26 and are joined to outer wall 38,forming channels 28 of a second fluid circuit. Channels 28 are definedby external wall 36 of channel 26, external fins 32, and outer wall 38.Fluid F₂ is delivered in an opposite direction from fluid F₁, asillustrated in FIG. 2 by the direction arrow depicting a flow of fluidF₂, in channels 28. Internal fins 30 in channel 26 augment heat transferbetween fluids F₁ and F₂. Channel 26 and channels 28 are fluidlyisolated. Multiple segments 25 can be joined to form a heat exchangercore. Outer walls 38 of adjacent segments 25 can be shared or integrallyformed.

Channel 26 extends longitudinally (i.e. axially). Channel 26 can have agenerally circular cross-section (i.e. channel 26 can be a cylindricaltube). In other embodiments, channel 26 can have other shapes tooptimize fluid flow dynamics and heat transfer. Channel 26 can beconfigured to transmit a cooling fluid and channels 28 can be configuredto transmit a heating fluid but in other embodiments the two may bereversed. For example, channel 26 can be configured to transmitsupercritical CO₂ and channels 28 can be configured to transmit air.Channel 26 is disposed such that channels 28 surround channel 26.

Channel 26 has internal fins 30. Internal fins 30 are integrally formedwith and extend from internal walls 34 of channel 26. Internal fins 30can be arranged in a spiraling orientation along internal walls 34.Channel 26 can have diameters and wall thickness designed for particularapplications. For example, channel 26 can be designed with wallthicknesses and cross-sectional diameters to accommodate pressurizedfluids (e.g., supercritical CO₂). In some embodiments, walls of channel26 can have a thickness greater than a thickness of external fins 32 toaccommodate pressurized fluids in channel 26.

Internal fins 30 can be disposed in channel 26 to increase convectivesurface area and augment heat transfer between fluids F₁ and F₂.Internal fins 30 can be integrally formed with internal walls 34 ofchannel 26 and can be arranged in a spiraling orientation along internalwall 24 of channel 26 along the flow length. Internal fins 30 can beconnected to internal walls 34 of channel 26 by a fillet. Theincorporation of internal fins 30 can increase surface area withoutaltering the external dimensions of channels 26 or channels 28. Theshape, size, and orientation of internal fins 30 have effects on flow,boundary layer, and heat transfer rate.

External fins 32 extend from and are integrally formed with externalwalls of 36 of channel 26 and outer wall 38. External fins 32 can beconnected to outer walls 36 of channel 26 by a fillet and connected toouter wall 38 by a fillet. External fins 32 wind or spiral aroundchannel 26. The arrangement of channels 26 and 28 and outer wall 38 insegment 25 can form a polygonal shape. As illustrated in FIG. 2A, across-section of segment 25 can be a hexagon but need not be limited tothat shape. The combined shape of channels 26 and 28 to form segment 25can vary depending on the number of external fins 32 provided onchannels 26. As illustrated in FIG. 2A, six external fins 32 extend fromchannel 26 to form six channels 28. In other embodiments, there may bemore or less channels 28 than those illustrated in FIG. 2A to optimizeheat transfer.

Channels 28 extend longitudinally and are defined by external walls 36of channel 26, external fins 32, and outer wall 38. Heat is transferredbetween fluid F₁ and fluid F₂ by external walls 36 of channel 26 andexternal fins 32. Adjacent channels 28 are separated by external fins32. External fins 32 form sidewalls of channels 28 and connect to outerwall 38, which forms an additional sidewall of channels 28. Externalfins 32 spiral around channel 26 along the length of channel 26 therebyforming spiraling channels 28. FIG. 2B is a perspective view of thefluid flow through segment 25 of FIG. 2A without channel 26 walls,internal fins 30, external fins 32, and outer wall 38. As illustrated inFIG. 2B, the fluid in channels 28 spirals around the fluid in channel26. Channels 28 can have cross-sectional areas and wall thicknessesdesigned for particular applications. For example, channels 28 can bedesigned with wall thicknesses and cross-sectional areas to accommodateto promote heat transfer.

The disclosed internal and external fin orientations and configurationscan provide improved heat transfer in counter-flow heat exchangerapplications. Internal fins 30 and external fins 32 are integrallyformed with channel walls to provide uninterrupted conductive heattransfer. The spiraling shapes of the internal fins 30 and external fins32 augment heat transfer by increasing the heat transfer coefficient.The spiraling shape creates a more turbulent flow which allows for heattransfer through both conduction and convection. The spiraling shape ofinternal fins 30 of channel 26 also provides advantage for two phasefluid flow by forcing liquid toward the channel wall while vaporcollects in the center, enhancing heat transfer. The shape andorientation of the internal fins 30 and external fins 32 allows foraugmented heat transfer without further modifying the externaldimensions of channels 26 and 28.

FIGS. 3A-3C illustrate different internal fin designs for use in channel12 of heat exchanger core 10 of FIG. 1 or channel 26 of heat exchangercore segment 25 of FIG. 2A.

FIGS. 3A-3C show different views of channels 40 with internal fins 42and optional internal fin 44 (shown in phantom). All channels 40 havethe same configuration. Channel 40 can be configured to transmit fluidF₁ in a flow direction illustrated by the arrow, substantially axiallywith respect to channel 40. FIG. 3A is a cutaway perspective view ofchannel 40. FIG. 3B is a perspective view of internal fins 42 and 44without the channel walls. FIG. 3C is an enlarged cross-sectional viewof channel 40 taken along the 3C-3C line of FIG. 3A. As illustrated inFIGS. 3A-3C, internal fins 42 are disposed in a spiraling orientationalong internal wall 46 of channel 40 and internal fin 44 is twisted anddisposed in a center of channel 40. The arrows in FIG. 3C indicate adirection in which internal fins 42 spirals along internal walls 46 anda direction in which internal fin 44 twists. FIGS. 3A-3C are discussedtogether.

As illustrated, channel 40 extends longitudinally (i.e. axially) and canhave a generally circular cross-section (i.e. channel 26 can be acylindrical tube). Channel 40 can be configured to transmit a heatingfluid. Internal fins 42 extend from and are integrally formed withinternal walls 46 of the channel 40. Optional internal fins 44 connectto and are integrally formed with internal fins 42.

Internal fins 42 are arranged in a spiraling orientation along internalwall 46 and can extend the length of channel 40. FIG. 3C shows fourinternal fins 42 spaced equal distances from each other along acircumference of channel 40. In other embodiments, there may be morethan four internal fins 42 or fewer than four internal fins 42. Internalfins 42 can also have unequal spacing along internal wall 46 of channel40 in other embodiments. Internal fins 42 extend into channel 40. Awidth of internal fins 42 or distance to which internal fins 42 extendfrom internal wall 46 into channel 40 can vary. In some embodiments,internal fins can have a width less than half a diameter of channel 40or less than one-third the diameter of channel 40, for example, asillustrated in FIGS. 3A and 3C. Internal fins 42 can provide a distinctadvantage for two-phase flow applications such as supercritical CO₂cycles. Internal fins 42 can drive the liquid phase of the fluid towardinternal walls 46, which promotes heat transfer, and allow the vapor,which is a less effective heat transfer medium, to collect in thecenter.

In some embodiments, internal fin 44 can be disposed in a center ofchannel 40 between internal fins 42. Internal fin 44 is optional and canbe excluded in some embodiments. Internal fin 44 can be twisted andextend the length of channel 40. Internal fin 44 is disposed betweeninternal fins 42 and is connected to internal fins 42 at varyinglocations along the length of channel 40. As illustrated in FIG. 3C,internal fin 44 can twist in an opposite direction of the spiralingorientation of internal fins 42. As internal fin 44 twists, it connectsto discrete locations on internal fins 42, as illustrated in FIG. 3A.

Channels 12 and 26 of FIGS. 1 and 2A can include other fin arrangementsas described in co-pending patent application Ser. No. ______ which isincorporated herein by reference in its entirety.

The spiraling and twisted shapes of internal fins 42 and 44 augment heattransfer by increasing the heat transfer coefficient. The shape ofinternal fins 42 and 44 also creates a more turbulent flow which allowsfor heat transfer through both conduction and convection. The twistedshapes provide advantages for two phase fluid flow by forcing liquidtoward the channel wall while vapor collects in the center, enhancingheat transfer. The shape and orientation of internal fins 42 and 44 inthis embodiment allows for augmented heat transfer without modifyingexternal dimensions of channels 12 and 26.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A heat exchanger includes a plurality of first channels extendinglongitudinally and a plurality of second channels. Each first channelincludes a plurality of first internal fins and a plurality of externalfins. The first internal fins extend from and are integrally formed withan internal wall and are arranged in a spiraling orientation. Theexternal fins extend from and are integrally formed with an externalwall. The plurality of second channels is defined in part by theexternal walls of the plurality of first channels and the plurality ofexternal fins. The plurality of second channels is fluidly isolated fromthe plurality of first channels.

The heat exchanger of the preceding paragraph can optionally include,additionally, and/or alternatively, any one or more of the followingfeatures and/or configurations:

A further embodiment of the heat exchanger of the preceding paragraphs,wherein the heat exchanger is additively manufactured.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein each first channel has a circular cross-section.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the plurality of external fins includes firstexternal fins and second external fins, wherein the first external finsconnect adjacent first channels.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the first and second external fins have anundulating shape extending a length of each first channel.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the second external fins protrude into the pluralityof second channels

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the first internal fins extend less than a fullwidth of each first channel.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the first internal fins are circumferentially spacedabout the internal wall and extend the length of each first channel.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein each first channel further includes a twisted centerfin.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the twisted center fin is disposed between and isconnected to the first internal fins.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein external fins of the plurality of external fins formsidewalls of each second channel and wherein the plurality of secondchannels further includes an outer wall, wherein the outer wall connectsthe plurality of external fins.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the external fins spiral around each first channelto form a plurality of spiraling fluid flow paths around each firstchannel.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the outer wall connecting the plurality of externalfins of each first channel has a polygonal shape.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein walls of the plurality of first channels have athickness greater than a thickness of the external fins.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the first internal fins extend less than a fullwidth of each first channel.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the first internal fins are circumferentially spacedabout the internal wall and extend the length of each first channel.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein each first channel further includes a twisted centerfin.

A further embodiment of the heat exchanger of any of the precedingparagraphs, wherein the twisted center fin is disposed between and isconnected to the first internal fins.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A heat exchanger comprising; a plurality of first channels extendinglongitudinally, each first channel comprising; a plurality of firstinternal fins extending from and integrally formed with an internal wallof each first channel, wherein first internal fins of the plurality ofinternal fins are arranged in a spiraling orientation along the internalwall; and a plurality of external fins extending from and integrallyformed with an external wall of each first channel, wherein theplurality of external fins includes first external fins and secondexternal fins, the first and second external fins having an undulatingshape extending a length of each first channel, wherein each firstexternal fin is disposed between second external fins, and where thefirst external fins connect adjacent first channels; a plurality ofsecond channels, each second channel defined by the external walls ofthe plurality of first channels and the first external fins, wherein theplurality of second channels is fluidly isolated from the plurality offirst channels and wherein and the second external fins protrude intothe plurality of second channels.
 2. The heat exchanger of claim 1,wherein the heat exchanger is additively manufactured.
 3. The heatexchanger of claim 2, wherein each first channel has a circularcross-section. 4-6. (canceled)
 7. The heat exchanger of claim 6, whereinthe first internal fins extend less than a full width of each firstchannel.
 8. The heat exchanger of claim 7, wherein the first internalfins are circumferentially spaced about the internal wall and extend thelength of each first channel.
 9. The heat exchanger of claim 8, whereineach first channel further comprises a twisted center fin.
 10. The heatexchanger of claim 9, wherein the twisted center fin is disposed betweenand is connected to the first internal fins.
 11. The heat exchanger ofclaim 3, wherein external fins of the plurality of external fins formsidewalls of each second channel and wherein the plurality of secondchannels further comprises an outer wall, wherein the outer wallconnects the plurality of external fins.
 12. The heat exchanger of claim11, wherein the external fins spiral around each first channel to form aplurality of spiraling fluid flow paths around each first channel. 13.The heat exchanger of claim 12, wherein the outer wall connecting theplurality of external fins of each first channel has a polygonal shape.14. The heat exchanger of claim 3, wherein walls of the plurality offirst channels have a thickness greater than a thickness of the externalfins.
 15. The heat exchanger of claim 13, wherein the first internalfins extend less than a full width of each first channel.
 16. The heatexchanger of claim 15, wherein the first internal fins arecircumferentially spaced about the internal wall and extend the lengthof each first channel.
 17. The heat exchanger of claim 16, wherein eachfirst channel further comprises a twisted center fin.
 18. The heatexchanger of claim 17, wherein the twisted center fin is disposedbetween and is connected to the first internal fins.